This invention relates generally to radio frequency (RF) power amplifiers (PA), and more particularly the invention relates to a solid state device used in RF PA having improved operating efficiency and reduced signal distortion.
Power amplifiers (PAs) are used when the efficiency and output power of an amplifying circuit are the important considerations. The various types of PAs are identified by their classes of operation, that is, Classes A, B, D, D, E, F, G, H, and S. Except for Class A, all of these amplifier types are easily differentiated from small-signal amplifiers by their circuit configurations, methods of operation, or both. There is no sharp dividing line between small signal and Class A power amplifiers; the choice of terms depends upon the intent of the designer.
Class A solid state power amplifiers are capable of providing highly linear amplification. However, they are considered cost-prohibitive for extremely high power transmitters (greater than 1 KW) because of fairly low power efficiency. As a result, for example, the TV transmitter industry has been forced to use bulky tube or Klystron amplifiers which require warm-up time, are narrow band, and are very inefficient in comparison to Class AB solid state power amplifiers. Class AB solid state power amplifiers do not have the high linearity and low distortion products that Class A solid state power amplifiers have. However, the power that they can provide is typically about 3-5 times greater, and their efficiency is far superior. A solid state device in a Class A power amplifier is always biased fully on, which means, in the case of a visual TV signal, it will pull the same current whether it is amplifying an all black signal (highest average power) or an all white signal (lowest average power). The transistor in a Class AB power amplifier is biased so that it is only slightly on. It will pull current in proportion to how it is being driven. When the amplifier is transmitting the highest average power, all black signal, it will pull the required current needed to get full power out. When transmitting a signal that has less average power (normal TV color pattern or an all white signal), a significantly less amount of current will be drawn. The use of solid state transistors in Class AB power amplifiers, which, generally, are more efficient than tube or klystron amps, greatly increases the overall efficiency of a television transmitter and reduce operating costs.
Power amplifiers built with power semiconductor devices, operating with the bias set in such a fashion as to operate in so called Class AB, exhibit amplitude and phase non-linearities that need to be corrected if the output is to reproduce the input with fidelity. These non-linearities lead to the production of unwanted signals at the output that need to be suppressed.
Because the bandwidth of a typical RF signal (such as a television visual signal) is usually a small fraction of the carrier frequency, the unwanted signals in the output of the power amplifier may be divided into three categories. FIG. 1 depicts the relationship of these signals to the desired signal, which consists in this case of two tones of equal amplitudes at frequencies fc.+-. fm. Amplifier non-linearity produces two types of unwanted signals, called harmonics and intermodulation distortion (IMD) products. The IMD products (identified as third, fifth, seventh, and ninth orders in FIG. 1) are prominent near the carrier frequency. They cause distortion in the received signal and/or adjacent channel interference. Other unwanted signals include parasitic and sub-harmonic oscillations and mixer products; they are called spurious products or simply "spurs". In an RF PA, the harmonics and some of the spurious products may be removed by filters; however, the IMD generated must be of an acceptably low level. Phase distortion (more predominant in Class AB amplifiers that employ bipolar transistors) will cause an uneven amplitude in the IMD products as shown in FIG. 2.
Present techniques to deal with amplitude and phase distortion in power RF amplifiers operating in Class AB is to predistort the input signal to compensate for the distortion introduced in the final amplifier by the use of a preconditioner amplifier. However, there is a limit to this technique. The need for amplitude linearity is quite obvious from FIG. 1, however the need for phase linearity is not so obvious. It can manifest itself, for example, in power amplifiers for the processing of television signals. When the image changes from black to white or vice-versa there is typically a 10 dB change in the power transmitted; if the insertion phase changes by more than 1.degree. it creates a "haze" around the edges of the white image that gives poor visual quality. Thus, there exists the need for a method to improve linearity, efficiency and stability in solid state transistors utilized in Class AB power amplifiers.